AS3, Using concatenated variables to access XML value

Hi
Im working on an Actionscript 3 program where i will click through a tree structure of buttons and use the values of the particular set of buttons, to access its corresponding value in an XML file.
As i click each button, I start forming a string eg.

Everything works perfect up to this point except that xmlString contains the incorrect value.
I want xmlString to contain the particular value within that particular element in the XML but instead it only displays the concatenated path. (myXML.unit1.lesson1)
It makes sense that it would display this value but my question is how would i be able to use that "xmlString" value to refernce the particular elements value.
Please note everything else works perfect and if i have to pass that XML path directly, "myXML.unit1.lesson1" it functions correctly, thus it must be an issue with the concatenated string not being registered as an XML path.
Im really so close to the finish line and I suspect this is very easy to do, so any help would be greatly appreciated.
Thanks JP